Audion amplifier



Jun 2.5, 1935.

P. O. FARNHAM AUDION AMPLIFIER Filed Aug. l0, 1955 rent control voltage.

"been proposed as a solution to this problem and,

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foutput audio wave for large signal input voltages other than the signal grid. More particularly,

Patented June 25, 1935 UNITED STATES PATENT OFFICE AUDION AMPLIFIER Paul 0. Farnham, Boonton, N. J., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application August 10, 1933, Serial No. 684,582

4 Claims. (Cl. Z50-20) This invention -relates to :amplifying systems and particularly to arrangements suitable for controlling the gain or output level of a voice Afrequency amplier by means o f a direct curdrawing, in which:

Fig. 1 is a somewhat schematic diagram of a embodying the invention;

Fig. 2 is a curve sheet showing the plate current ow in the amplifier tube for different values of the negative bias applied to the gain control grid; and

Fig. 3 is a fragmentary circuit diagram ofA another embodiment of the invention.

In the drawing, the reference character RF identities a radio amplifier including one or more tube stages for amplifying a desired signal at the `received carrier `frequency and/or an interme- The invention is especially useful in connection with the problem of controlling the audio output level of a radio receiving system of the automatic gain control type from a point remote .from the audio amplifier. Various methods have in my copending application Serial No) 671,771, led May 18, 1933, I have described.l and claimed a method of audio level control which contemplates the control of radio gain in a stage following the automatic gain control system. Other methods involving the use of gain control attenuators or passive networks such as potentiometers4 involve expensive impedance adjusting networks or, alternatively, complicated and expensive motor systems if the actual control point is located remotel from the audio amplier. Some attempts have been made to control the gain of `an audio amplifier stage by varying the direct current bias between the signal input grid and the cathode of a vacuum tube but this method leads to considerable distortion of the be controlled automatically by a gain control system AGC to maintain an approximately constant lradio input to the detector stage D;

The tube I of the audio stage working out of the detector has at least three grids seriallyarranged between cathode K and the lplate P, and the audio output of the detector is impressed upon the outer or third grid G3 through an appropriate The grid G3 is placed at a desired negative potential Ea with respect to cathodeK by a circuit including a bias resistance 3 and a tap 4 which is and high negative grid biases. The use of a adjustable along a direct currentrsource 5. here space-charge tube has also been attempted as one solution to this audio volume control problem, i. e., the signal input was applied to an outer negatively polarized grid while an inner grid polarized with a variable direct current potential positive to the cathode was usedto control the gain of the stage. This latter method suffered from the distortion introduced when the posi- `tive potential on the space charge grid was lowered to obtain a substantial reduction in gain, and was also inadequate from the standpoint of obtaining a suiiiciently wide range of control.

An object of the invention is to 'provide novel methods and circuit `arrangements `for vcontrolling the gain of an audio amplifier by adjustment of a direct current bias applied to a grid tion as a battery. The resistance 6 of a potentiometer is shunted across the current source 5 andthe tap 1 is connected to the inner grid G1 to place a negative bias on that grid. As indicated by the breaking away of the leads to the potentiometer, this gain control unit may be located at some distance from the controlled tube. Condensers 8 are provided to by-pass the leads for signal frequencies when the potentiometer is located at a remote control point, thus insuring that there will be no pickup and transmission of undesired currents of audible frequencies to the controlled stage. The intermediate grid G2 is maintained at a desired positive potential E2 with respect to the catho-de by a tapped connection u9` to the plate current source, battery l0. The plate circuit includes Va resistance ll or other appropriate impedance to which a succeeding audio amplier AF is coupled, the amplifier AF working into a loud speaker or other appropriate form ofh reproducer R.

The method of operation and advantages of the invention will be apparent from a consideration of the family of curves showingthe relation of plate current, ip, to the signal grid bias, Es. Curve` an object is to provide an audio amplifier including a tube having atleast two negatively polarized grids with an intervening positive grid and an outside anode, and circuits for impressing the signal voltage on one negative grid and an adjustable gain control bias `upon the other negative grid.

'Ihese and other objects and advantages of the invention will bey apparent from the following shown diagrammatically for purposes of illustra.

specication when taken with the accompanying A radio receiver including an audio amplier stage diate frequency. The gain of the 4amplifier may coupling, such as that provided by condenser 2, 25

A is the plate current grid bias characteristic obtained when the bias voltage E1 on the inner grid was reduced to zero, and this will be recognized as having the general form of the characteristic of the usual amplier tubes. Curves B, C and D are characteristic curves obtained when the Voltage E1 on. the inner grdhad Values 0f -5, 1o and "15 i/bits, respetiveiy. Pemex jen curve 'A is the normal operating lpoint for maximum gain and, as is well known, the stage gain may be reduced by making the signal grid bia'sEs more negative, for example by shiftingthemoperating point to one of the positions indicated by X1, X2, X3. The stage gain foran'iinpr'essd signal voltage E is proportionall to the transconductance or slope of the'curveat the operating point.

While the distortion of the signal is pro-portional to n da, 11:32

is therefore apparent that the Vdistortion increases Ywhen the bias E3 on the signal grid is adjusted to reduce the stage gain.

'I'he operating point may be shifted to another curve by adjustment of the negative bias E1 on the inner' grid and, if bias E3 remains hired, the operating point will moveV downward along the ordinate through point X to the curve B, C, etc. for the selected value of the inner grid bias E1. The series of operating points :111, rc2, :Us thus obtained by adjustment of E1 are located at portions of curves B, C, D, respectively, which have less slope and'less curvature than curve A at point X. For purposes of comparison, the points X1, X2 and X2 were located at those regions on curve A at which the slope was the same as that of curves B, C, D at points :171, :132, :133, respectively. The stage gain for small inputs will therefore be the same when operating at either of the pairs of points, X1, :U1 or X2, :152, etc., but the distortion is substantially less when a xed signal grid bias is used and the gain vis controlled by adjustment of the inner grid bias E1.

Solong as the operating'point is shifted along curve A by adjusting the signal grid bias, a del crease in transconductance is obtainable only with an increase in distortion, While by shifting the operating point across the family of curves by different bias voltages E1, the transconductance is Y with lamentary cathodes by adjustment of the heating current and is, in fact, based upon a variation of the electron stream in which the signal grid G3 is located. The positively polarized grid G2 attracts electrons from the cathode K and those electrons which are thus attracted and pass grid G1, So far as concerns the control action of thefsignal'g'rid G3, the electron source is not the cathode I; but is a virtual cathode which may be considered as existing in the region of the grid G2.

While this type of control may be obtained with the usual pentode tubes having an outer grid of relatively coarse mesh or long pitch,

a grid G3 of relatively fine mesh is preferably' employed to obtain higher maximum gain for the signal frequencies and, if desired, one or more turns may be omitted from this grid Winding to bt'ai'n Va variable `mu characteristic.

For a'control over a wide 'range di input Voltages, it may be desirable to increase the signal grid bias E3, as E1 is increased to lower the gain, anduat any appropriate rate with respect to the adjustment of E1. rThe locus of the operating points whenv both bias voltages are varied may be, foreX'ampla'as indicated by the dotted line XY. Vifhenised"as an audio stage in a receiver of the automaticg'ain control type, it is not necesjsary, in/Ageneralfto make any adjustment 4of the bias on the signal grid since the input to the detector will be maintained approximately constant by the automatic gain control system and the'adjstment of the bias on the inner grid `provides v'a lsi'lificient range oi control to bring the receiver output to any desired level.

As shown in Fig. '3, the controlled stage may include a tube l of the type known commercially` as 2Al,` which tube is ordinarily used as anoscillator-rst detector combination in superheterodyne receivers. This type of tube includes five grids, the grids G3 and G5 being connected within the tube and brought out to a single tube terminal. Except that the signal voltage is applied to grid G4, the circuit connections are, or may be, substantially as shown in Fig. 1, and will not be again described in detail. Grids G3, G5 are connected to a desired point of positive potential on the source Ill through a resistance I2,

and the grids are by-passed to the cathode for all signal frequencies by a condenser i3. While the resistance l2. is not essential, it is desirable to empl-oy one of such value that the reduction in space current ow to these grids as the control bias E1 is made more negative increases the cathode to grid voltages (by the decreased potential vdrop across resistance l2) to eiect a further reduction in the curvature of the ip vs E4 characteristic.

As in the case of the Fig. 1 circuit, the bias voltage Eton the signal Grid G4 may be made somewhat more negative as the gain control bias E1 is increased negatively.

It is apparent that the invention may be applied as Well to the control of gain in a radio irequency stage. It is of particular advantage, however, for the remote control of gain in an audio amplifier such 'as might be used in a radio receiver employing automatic gain control, in a public address system or in an electrical system for phonographic reproduction. The tube structure beyond the virtual cathode formed by the inner negatively polarized grid and the adjacent positively polarized grid may be or" any desired type to constitute a triode, tetrode or other type of amplier.

I claim:

1. In the operation of an amplifier stage lincluding a tube having a cathode cooperating with a signal grid and anode, the method of reducing the transconductance'of the tube from signal grid to anode without increasing the distortion intron duced into the amplied output of the tube, comprising the establishing of a positive or accelerating eld between the signal grid and cathode, controlling the electron ow from said cathode to the region ofthe positive eld and signal grid by a negative eld adjacent said cathode, initially impressing on the signal grid such a negative bias which results in substantially maximum amplification and increasing the signal grid bias in a negative sense as the negative field is increased in magnitude to reduce the stagegain.

2. An ampliiier stage comprising a vacuum tube having a cathode, a positively polarized anode and three grid electrodes between said cathode and anode, means impressing a negative bias on the outermost and innermost of said three grids, means polarizing the intervening grid positive with respect to said cathode, circuit connections impressing a signal voltage between the outer of said grids and cathode, an output circuit between said anode and cathode, means for adjusting the negative bias on the inner negative grid, a fourth grid located between the outermost of said grids and anode, a direct current circuit between said fourth grid and cathode including a resistance and a current source establishing a positive potential on said fourth grid; said resistance having a magnitude such that space current iiow in said direct current circuit produces a substantial potential drop across the same when the negative potential on said inner grid is adjusted for maximum gain.

3. An amplifier stage comprising a tube having a cathode cooperating with a signal grid and an anode to form an amplifier, an input circuit connected betwen said signal grid and cathode, an output circuit between said anode and cathode, means for controlling the gain of the stage and comprising a pair of oppositely polarized electrodes located between said cathode and signal grid, and means including a potentiometer located at a point remote from said tube and a contact adjustable therealong for adjusting the potential between one of said electrodes and the cathode.

4. 'I'he combination defined in claim 3 in which said potentiometer is connected to said tube by a pair of long leads and a capacitor having a low impedance to the signal frequencies to be amplified is 'connected across said leads adjacent said tube whereby undesired induction currents picked up by said leads are prevented from reaching said electrodes.

PAUL O. FARNHAM. 

